JP2009521142A - Transducer operated by sound wave and filter having the transducer - Google Patents

Abstract

Described here is a SAW converter that suppresses an obstructing transverse mode. Here, this is achieved by matching the transverse excitation profile of the sound wave with the transverse fundamental mode of the waveguide formed by the acoustic track and the adjacent outer region (AU1, AU2). This adaptation is performed by dividing the acoustic track into an excitation region (MB) and edge regions (RB1, RB2). The edge region and the outer region are configured such that the longitudinal phase velocity of the sound wave in the edge region is larger than that of the excitation region, and the longitudinal phase velocity in the outer region is larger than that of the excitation region. small. _Ky is a real number in the edge region and an imaginary number in the outer region. By adjusting the width of the edge region (RB1, RB2), k _y in the excitation region of the is substantially constant and so that substantially equal to zero.

Description

  Described here is a SAW transducer, ie, an electroacoustic transducer operating with surface acoustic waves (SAW = Surface Acoustic Wave). SAW converters are used, for example, in portable mobile radio devices.

  In a SAW converter, an electrical signal is converted into a sound wave and vice versa. When a sound wave propagates in the SAW converter, a diffraction loss occurs due to a part of the surface wave being radiated laterally in the edge region of the converter. A method for suppressing higher order transverse modes or adapting the transducer excitation profile to the shape of the transverse fundamental mode is known, for example, from the publication DE196 38 398 C2.

  Furthermore, from publications DE 10331323A, EP 1,471,638 A2 and US 5,121,860 another SAW transducer is known which has regions in the transverse direction where the speeds of sound waves differ from one another.

  An object of the present invention is to provide a transducer that operates on a surface acoustic wave, in which a transverse mode that is an obstacle is suppressed.

  The electroacoustic transducer has a piezoelectric substrate and electrode fingers arranged on the piezoelectric substrate, and the electrode fingers form an electrode lattice for exciting sound waves.

  Here, a transducer having an acoustic track is provided. This acoustic track is arranged on a piezoelectric substrate. The acoustic track includes intermeshing electrode fingers that are connected to the first and second busbars substantially alternately. A surface wave can be excited between electrode fingers having different polarities, and this surface wave can propagate in the acoustic track.

This acoustic wave propagates in one plane. This plane is referred to as an x, y plane. In the two-dimensional space formed by the x and y planes, the wave can be characterized by a wave vector (k _x , k _y ) having a longitudinal component k _x and a transverse component k _y . These components k _x and k _y are also referred to as longitudinal or transverse wave numbers.

The acoustic track is arranged in the lateral direction y between two external areas adjacent to it. In this outer region, the waves described above are preferably substantially attenuated or cannot propagate. The amplitude of this wave decreases exponentially in the lateral direction indicated by the track in the outer region. Said external region can usually be formed by an empty substrate surface or an at least partially metallized substrate surface. Outer region in an advantageous variant embodiment, can be a metal stripe which extends in the x-direction, a width of, for example, at least λ _{y =} 2π / k _y.

The acoustic track has one excitation region and two edge regions, and the excitation region is disposed between the edge regions. The edge region and the outer region are configured such that the longitudinal phase velocity of the sound wave in the edge region is larger than that of the excitation region, and the longitudinal phase velocity in the outer region is larger than that of the excitation region. Is also small. _Ky is a real number in the edge region and an imaginary number in the outer region. The wave is preferably excited only in the excitation region, i.e. not in the edge region, but it can propagate in this edge region as well as in the excitation region. This is because _ky is a real number here.

The width of the edge region is advantageously adjusted as follows. That is, the absolute value of k _y in the excitation region, the advantage over the edge region and the outer region is being adjusted to be significantly (e.g. at least 10 times) smaller. With the acoustic track structure described above, and in particular by appropriately selecting the width of the edge region, k _y = 0 is advantageously achieved in the excitation region.

  With the transducer structure described above, the acoustic modes (main modes) to be excited can be bound in the acoustic track or substantially in the excitation region.

  The entire area and the external area of the acoustic track extend in the longitudinal direction x and thus in parallel with each other. The wave velocity in the excitation region is greater than the outer region, but smaller than the edge region of the acoustic track. The above converter has a structure of an inverse wellenleister in the transverse direction y. The acoustic track forms the core of this waveguide, while the outer region forms its sides.

The waveguide region is a region selected from the first outer region, the second outer region, and different regions of the acoustic track, that is, the excitation region and the first and second edge regions. (K _x ) ² + (k _{y, i} ) ² = (ω / v _i ) ² holds in the selected i-th waveguide region where waves can propagate. ω is the angular frequency of the wave, v _i is the propagation velocity of the wave in the i-th waveguide region. k _{y, i} is the wave number in the horizontal direction in each waveguide region.

  The acoustic wave is characterized by a transverse fundamental mode. The transverse fundamental mode is obtained from the transverse velocity profile of the waveguide formed by the acoustic track and the outer region. Here, most of the energy of the sound wave in the acoustic track is concentrated in the acoustic track.

  At least a part of the first bus belongs to the first external area, and at least a part of the second bus belongs to the second external area.

  Corresponding to the periodic arrangement of the electrode fingers of the transducer in many cases, sound waves propagate mainly on both sides in the longitudinal direction. However, it is also possible for this transducer to advantageously radiate the excited sound waves only on one longitudinal side. Such a converter can be used, for example, in a recursive filter.

The propagation speed of the surface acoustic wave excited in the acoustic track is reduced (compared to an empty substrate surface) by metallizing the substrate surface in many piezoelectric substrates such as quartz, LiNbO ₃ , LiTaO ₃ .

  Hereinafter, the lattice-shaped arrangement configuration refers to a periodic arrangement configuration of metal stripes extending in the lateral direction. Instead of this metal stripe, basically any acoustic inhomogeneity is advantageous, for example a groove, in order to adjust the speed of the sound waves.

  In the above-described excitation region, the electrode fingers advantageously have a grid arrangement. In the transducer region where no excitation takes place, for example in the edge region, this grid arrangement can be formed by a plurality of electrode finger regions or by placing a stubmelfinger, where These electrode finger regions continue to each other, are at the same potential, and are adjacent to the bus. This lattice arrangement can also be configured as a perforated metal stripe.

  In an acoustic track with a periodic grid arrangement, the surface wave velocity (if the proportion of metallized surface in each transducer region is the same) reduces the spacing between the centers of the grid structures (stripes). Decreases with it. This is because the wave is further decelerated as the grating period becomes smaller at the end of the finger. It is therefore advantageous to obtain a higher velocity in the edge region than in the excitation region by selecting the spacing between the periodically arranged metal stripes to be larger than the excitation region.

  Furthermore, the surface wave velocity depends on the metallization ratio, ie the proportion of the surface metallized in each transducer region. By increasing the proportion of propagation sections that are metallized, the wave velocity decreases with increasing metallization ratio (assuming the period of the lattice arrangement remains the same). Thus, it is advantageous if a speed greater than the excitation region is achieved in the edge region by choosing the average metallization ratio in the edge region to be smaller than the excitation region. In the outer region described above, a lower speed than in the excitation region can be achieved by selecting an average metallization ratio in the outer region greater than in the excitation region. The entire metallization outer region is particularly advantageous. It is also possible to take other measures to reduce the speed in the outer area.

  If there is an angular region around the main propagation direction in which surface waves can propagate in the acoustic track, the SAW track can act as a waveguide in the lateral direction with the outer region adjacent to it. Here, the waves are advantageously totally reflected at the boundary with the outer region at the same time, so that no losses due to lateral radiation outside the acoustic track occur.

  The transducer comprises in one variant embodiment a track device, which comprises a plurality of SAW tracks that are electrically connected to one another and are preferably arranged parallel to one another. In addition, this track device can act as a waveguide in the lateral direction together with the external region adjacent to the track device if the conditions for the above-described acoustic track are satisfied.

In the following, the deviation of the input coupled sound wave is referred to as excitation intensity (Anregungsstaerke). The acoustic track described above is characterized by the excitation intensity (in the longitudinal or transverse direction). This excitation intensity is proportional to the potential difference ΔU between the electrode fingers arranged in the vertical direction of different electrodes. Here, these electrode fingers together form a pair of fingers for excitation. Here, the excitation intensity depending on the lateral coordinate Y is referred to as excitation profile Ψ _y .

を使用することができ、ただし、例えばα＝０．９であり、また有利にはα＝０．９５が選択される。Φ_ｙは横方向座標Ｙに依存する横方向の基本モードの偏差である。 In the waveguide formed in this way, a plurality of transverse modes (fundamental modes and their harmonics) of sound waves can be excited or propagated. Here, if the acoustic track is configured laterally and the corresponding lateral excitation profile Ψ _{y of the} wave is adapted to the fundamental mode shape Φ _y , the electrical signal is Maximum input coupling to the basic mode. Here, as a judgment condition for this conformity, a relational expression

Can be used, however, for example, α = 0.9 and advantageously α = 0.95 is selected. Φ _y is a deviation of the fundamental mode in the lateral direction depending on the lateral coordinate Y.

  When the electrical signal is optimally coupled to the fundamental acoustic mode, the coupling to higher order modes disappears. This is because a system composed of a plurality of transverse modes is approximately orthogonal.

The width of each edge region in the transverse direction is advantageously a λ _{y /} 8~λ _{y /} 4, where lambda _y is the wavelength of the fundamental mode of transverse direction in the edge regions corresponding.

The absolute value of k _y in the edge region, since much greater than the excitation region, the deviation of the transverse modes in the horizontal direction is correspondingly rapidly changing at the edge region. Since the transverse wavenumber value ky in the outer region is imaginary and its absolute value is much larger (eg at least 10 times) than in the excitation region, this ensures fast decay of the transverse mode in the transverse direction. The Thus, an approximately rectangular fundamental mode can be produced in the waveguide, the slope of the side edge of this fundamental mode being dependent on the absolute width of the edge track, and ultimately the edge region, Depends on the difference in wave phase velocities in the excitation and external regions

  Obstructive transverse wave modes are suppressed by improving the electrical signal input coupling to the transverse acoustic fundamental mode by a special configuration and placement of the edge region of the acoustic track.

  The advantage of the above converter that suppresses the disturbing transverse wave modes is that when designing such a converter, the simulated transfer function of this converter matches the actual transfer function well. Therefore, it is sufficient to perform a simulation of wave propagation in only one direction (longitudinal direction). Here, a complicated simulation of a two-dimensional (longitudinal and transverse) wave propagation action can be omitted.

  Dividing the acoustic track into one excitation region and two edge regions as described above is different from dividing the track into a plurality of partial tracks as is well known in the following respects. That is, the difference is that the sound wave is not excited in the longitudinal direction in the edge region of the transducer, and the wave excited in the excitation region is accelerated as expected.

  The above edge region is only used to tune the transverse waveguide fundamental mode (different from a sine wave) by setting the appropriate velocity profile of the waveguide. To adjust the shape of the transverse fundamental mode, for example, the width of the edge region and / or the phase velocity of the waves can be changed.

  In order to adapt the shape of the fundamental acoustic mode as close to a rectangle as possible, the wave velocity in the edge region is much higher than the excitation region for the transducer with concave slowness described in connection with FIGS. 9A and 9B. It is advantageous if the velocity in the outer region is much smaller than in the excitation region. The velocity in the outer region is preferably less than the excitation region by at least 2%, preferably by at least 3%. It is also possible to obtain a difference of 5% or more. The velocity in the edge region is preferably greater than the excitation region by at least 2%, preferably by at least 3%.

  The reduction in velocity in the outer region is achieved by the metallization ratio as large as possible, most advantageously by metallization over the entire surface of the base piezoelectric substrate. Also, by increasing the thickness of the metal in the outer region than the acoustic track, the speed in the outer region is further reduced.

  The reduction in surface wave velocity to be achieved in the outer region can also be obtained as follows. That is, it is also possible to obtain metallization with reduced stiffness or increased density compared to the acoustic track described above, in particular compared to the excitation region, in the outer region. For example, in the case of a converter having an electrode containing aluminum in the external region, a layer made of gold, platinum, copper or a layer sequence thereof can be used in addition to the Al layer. It is also possible to use a layer sequence of any material, preferably a relatively low stiffness or a relatively high density material in the outer region.

  In order to increase the speed in the edge region, a periodic lattice arrangement with a longer period than the excitation region can be used. In this case, it is possible to select a metallization ratio in the edge region smaller than that in the excitation region. The metallization ratio in the edge region and the excitation region of the acoustic track can be the same in another variant embodiment. If the period of the lattice arrangement is the same, the metallization ratio in the edge region can be selected smaller than that in the excitation region.

  As the edge region, for example, the lateral gap of the above-mentioned transducer can be used. The lateral gap is a region extending in the lateral direction between the end of the finger and the opposing busbar or stub-like finger. Since this region lacks fingers compared to the excitation region, the average period is increased and the metallization ratio is decreased. In this case, the rectangular wave profile can be adjusted by the width of the edge region.

  Each of the above edge regions can also be realized as a partial track, where the period and the metallization ratio are advantageously selected for the speed to be achieved. The electrode fingers in the edge region are preferably arranged in a periodic grid.

  In the above edge region, it is possible to increase the speed further by using a material that is stiffer or less dense than the acoustic track for the periodically arranged stripes.

  For example, aluminum can be used in the edge region for a transducer having an electrode containing copper in the excitation region.

  In all known approaches, the excitation profile of the transducer is adapted to the lateral fundamental mode.

  In an advantageous variant embodiment of the transducer described above, the excitation profile of the transducer can additionally be fine-tuned to adapt to the shape of the transverse fundamental mode defined above.

This fine-tuning adaptation can be realized, for example, by dividing the excitation region into a plurality of partial tracks in the lateral direction, where each partial track forms a partial converter. These partial tracks or partial converters are connected in series and / or in parallel. The series connection reduces the potential difference between the electrode fingers that perform excitation, thereby reducing the excitation intensity in the partial track. These partial tracks have the same configuration in the vertical direction except for the width. Here, the width of the partial track is selected so that the transverse profile Ψ _y of the excitation intensity in the excitation region matches the shape Φ _y of the transverse fundamental mode.

Below, based on an Example and the figure corresponding to this, said converter is demonstrated in detail. These figures are schematic and not to scale. The same reference numerals are assigned to the same parts or parts having the same function. Here, FIG. 1 schematically shows the transducer, the change in wave number in the transverse direction, the shape of the corresponding fundamental mode and the transverse velocity profile,
FIG. 2A schematically shows a transducer in which an edge region is formed between the end of the electrode finger and the bus bar facing it.
FIG. 2B schematically shows a transducer in which the edge region is configured as a perforated busbar region,
FIG. 3 schematically shows another transducer (lower) in which the excitation region is divided into partial tracks connected in series with each other, the corresponding lateral excitation profile, and the shape of the transverse fundamental mode (upper). Has been
FIG. 4 shows another transducer (lower) in which the excitation region is divided into partial tracks connected in series and in parallel to each other, the corresponding lateral excitation profile and the shape of the transverse fundamental mode (upper). Is schematically shown,
FIG. 5 schematically shows another transducer (bottom) in which a plurality of acoustic tracks are connected in sequence, the corresponding transverse fundamental mode, and the change in wavenumber in the transverse direction (top). ,
FIG. 6A schematically shows a lateral profile of the metallization height in a transducer with a thicker outer region compared to the excitation region,
In FIG. 6B, the lateral profile of the metallization height in a transducer with the outer region thickened by an additional material layer is shown schematically,
FIG. 7a) shows the deviation of the transverse wave modes that can propagate in the acoustic track if the transverse excitation profile is not adapted, and b) shows the excitation intensity corresponding to these modes. Shown,
FIG. 8 (a) shows the deviations of the transverse wave modes that can propagate in the acoustic track when the transverse excitation profile is adapted to the fundamental modes, and FIG. 8 (b) shows the excitations corresponding to these modes. The strength is outlined,
FIG. 9A schematically shows a slowness curve in a waveguide having a convex slowness,
FIG. 9B schematically shows a slowness curve in a waveguide having a concave slowness.

In FIG. 1, for example, a SAW transducer having an acoustic track AS arranged on a piezoelectric substrate such as 42 ° YXLiTaO ₃ and capable of propagating acoustic surface waves in the longitudinal direction X, depends on the lateral coordinate Y. The square of the wave number k _y of the transverse mode, the transverse mode Φ _y (upper side) obtained from the _ky profile, and the lateral velocity profile (lower side) are shown.

The acoustic track AS is divided into one excitation region MB and two edge regions RB1 and RB2. The width of the edge region in the transverse direction is advantageously a λ _{y /} 8~λ _{y /} 4, where lambda _y is the wavelength of the transverse fundamental mode in the edge region.

であり、またＡおよびＢは係数である。 The above wave number k _y indicates that the lateral deviation Φ _y of the fundamental mode depends on the lateral coordinate Y in each of the lateral regions MB, RB1, RB2, AU1, and AU2,

And A and B are coefficients.

  The above transducer has two electrodes, each of which includes one bus E1, E2 and electrode fingers connected thereto. The buses E1 and E2 are relatively wide and metallized regions throughout. In an advantageous embodiment, the height of the metallization here is larger than the acoustic track AS. See FIGS. 6A and 6B for this.

The height profile of the transducer along the transverse direction Y is shown schematically in FIG. 6A. The height direction Z is oriented in a direction perpendicular to the surface of the substrate. Excitation area MB and the edge area RB1, RB2 metal structures are arranged in part has a thickness or height h _1. In the outer region, an additional metallization of the same material, for example aluminum or an aluminum alloy, is deposited with a thickness h ₂ -h ₁ . This additional metallization reduces the speed in the outer area.

The above transducer structure, for example, the electrode fingers in the excitation region MB, the periodic gratings in the edge regions RB1, RB2, and the structures belonging to the external regions AU1, AU2 (eg, buses E1, E2) are shown in FIG. 6B. , it is formed in the metal layer S1 of thickness h ₁ which is deposited on the piezoelectric substrate. Here, the same electrode material, for example aluminum or an aluminum alloy, is used in the layer S1 in all regions.

  In FIG. 6B, an additional layer S2 is deposited on this layer in order to thicken the metal structure comprised in the layer S1 in the outer regions AU1, AU2. Here, this layer S2 preferably has a greater density and / or less stiffness than the material of the layer S1. For example, gold, platinum or copper is advantageous for the layer S2. In this case, even when the overall thickness of the structures arranged in the external areas AU1 and AU2 is relatively small, a relatively large speed reduction can be obtained.

In the embodiment of FIG. 1, the external areas AU1, AU2 are configured such that the velocity of the surface wave in these areas is lower than that of the excitation area MB. Width of the busbars, preferably at least 10 [lambda] _x. The electrode fingers of the different electrodes are alternately arranged in the excitation region to form a pair of fingers that perform excitation. The excitation profile of the acoustic track AS depends on the excitation area and is rectangular in this embodiment.

  All electrode fingers in the edge region belong to the same electrode and are therefore inactive. That is, sound waves are not excited in this edge region. The waves are not excited in the edge regions RB1 and RB2, but here the waves are induced by the excitation in the excitation region.

In this embodiment, the edge region has a lattice-like structure, where the period of this lattice is selected to be larger than the average grid of the excitation region MB. Since the above-mentioned wave encounters fewer edges of the grating in the edge regions RB1 and RB2 than in the excitation region MB, the phase velocity v of the sound wave increases in these edge regions. Furthermore, the fact that the metallization ratio in the edge region is smaller than that in the excitation region contributes to an increase in the wave velocity. Therefore, the velocity v _RB in the edge regions RB1 and RB2 is larger than the velocity v _MB in the excitation region MB. . On the other hand, the external areas AU1 and AU2 are configured as follows. That is, the speed v _AU in the external areas AU1 and AU2 is configured to be smaller than the speed v _MB in the excitation area MB.

によって特徴付けられる。束縛される波モード（gebundene Wellenmode）に対して横方向の波数ｋ_ｙは、上記の導波体のコア領域（すなわち音響トラックＡＳ）内で実数であり、導波体の被覆領域（外部領域ＡＵ１，ＡＵ２）においては虚数である。 The acoustic track AS and the metallized outer regions AU1, AU2 which are in contact with the acoustic track in the lateral direction together form a waveguide. Transverse waveguide mode is phase coefficient

Is characterized by Wave number k _y transverse to bound the wave mode (gebundene Wellenmode) is a real number in the core region of the waveguide body (i.e. acoustic track AS), covered areas (outer area of the waveguide AU1 , AU2) is an imaginary number.

The absolute value of _{k y} in the excitation region MB is a region RB1, RB2, AU1, significantly (e.g. at least 10 times) than AU2 small. If k _y = 0 (in the excitation region), the fundamental mode has a plateau in this region. That is, the wave deviation in the excitation region is constant in the lateral direction Y.

In acoustic track is external to the AS and the outer region that this the contact laterally AU1, AU2, _{k y} is the imaginary or _(k ^y) 2 <0. Therefore, the amplitude of the waves in the outer areas AU1 and AU2 decreases exponentially in the lateral direction.

The wave number k _{y in the} lateral direction is a real number or (k _y ) ² > 0 in each of the edge regions RB1 and RB2. In these edge regions, it varies from a maximum amplitude in the excitation region to a small amplitude at the boundary with the external region.

The value of the wavelength λ _y in the edge region depends on the wave propagation speed in the longitudinal direction, and this propagation speed itself depends on the grid of electrode fingers in the edge region. The absolute width of this edge region can be variously selected (depending on a predetermined value of λ _y ). The width of the edge region measured in wavelength is preferably λ _y / 4 to λ _y / 8. By changing the absolute width of the edge region, the slope of the corresponding side edge in the fundamental mode can be adjusted. The shape of the transverse fundamental mode is determined by the width of the edge region selected as described above, and in this shape, the wave amplitude decreases exponentially outward in the outer region and also in the edge region. A standing wave is formed in the direction. Here, the antinodes of standing waves are at the edges of the excitation region and the edge region. This wave travels up to a quarter of the sine wave in the edge region. This wave decays to zero in the outer region, so there is no zero deviation in the edge region. For this reason, it is advantageous that the width of the edge region is λ _y / 4 or less. The width of the edge region is preferably λ _y / 8 to λ _y / 4. Particularly advantageous is the width of the edge region which is substantially λy / 4. This is because this will cause the waves to enter the outer area only slightly. This allows the fundamental mode shape to be maximally adapted to the rectangular excitation profile shape of the acoustic track AS.

から決定することができ、ただしｋ_ｙ，ＲＢは、縁部領域における横方向波数であり、ｋ_ｙ，ＡＤは、外部領域における横方向波数である。ここで仮定したのは、励起領域においてｋ_ｙ≒０であることである。すなわち、縁部領域に波がわずかに進入することは、比｜ｋ_ｙ，ＡＵ｜／ｋ_ｙ，ＲＢの値が大きいことと同義である。有利には上記の変換器領域を構成して、
｜ｋ_ｙ，ＡＵ｜／ｋ_ｙ，ＲＢ≧１
が成り立つようにする。 The width W of the edge region is, for example, an expression

Where ky _{, RB} is the transverse wavenumber in the edge region and _{ky, AD} is the transverse wavenumber in the outer region. The assumption here is that k _y ≈0 in the excitation region. That is, a slight wave entering the edge region is synonymous with a large value of the ratio | ky _{, AU} | / ky _{, RB} . Advantageously, the above-mentioned converter region is configured,
| Ky _{, AU} | / ky _{, RB} ≧ 1
Make sure that

The more the wave number k _y is a number in the edge region, the corresponding wavelength becomes smaller, and thus the absolute width of the edge area is reduced. If the _ky value is large, the side edge slope of the transverse fundamental mode will be correspondingly large.

The above electrode fingers are actually perpendicular to the longitudinal direction X, which is the main propagation direction of the wave. In the ideal case where the finger length is infinite, the sound wave propagates in the main propagation direction. Due to the finite size of the excitation region of the acoustic track, the above propagation is also performed in an angle range −θ _max <θ <θ _max in the direction deviating from the main propagation direction. θ is an angle formed between the propagation direction and the main propagation direction. The wave velocity v is determined by the dependence of the angle range on the angle θ whether or not the transducer can function as a waveguide for this wave.

の成分である。

は波数ベクトルである。 Important to this is the behavior of the curve s _y (s _x ) shown in FIGS. 9A and 9B at the core and sides of the waveguide. s _x = k _x / ω and s _y = k _y / ω are the slowness vectors in the x or y direction

It is a component.

Is a wave vector.

The slowness component s _y includes a real part Re {s _y } and an imaginary part Im {s _y }. Here, Re {s _y } / s _x = tan (θ) holds. This curve is also called slowness (in English, “slowness”). This is because this shows the behavior of the reciprocal speed with respect to the angle θ.

By R _MB and R _AU , the slowness curve Re {s _y (s _x )} in the excitation region MB or the outer region AU1, AU2 of this converter, ie, the slowness component s _y plotted against s _x in each region Indicates a real value. With I _MB and I _AU , the sloWness curve Im {s _y (s _x )} in the excitation region MB or the outer region AU1, AU2 of this converter, ie, the imaginary value of the slowness component s _y plotted against s _x is obtained. Show.

A distinction is made between convex slowness and concave slowness. The convex slowness (FIG. 9A) means that as the absolute value of θ increases, the component s _{x of the} slowness vector decreases and the absolute value of the component Re {s _y } increases. That is, as the absolute value of θ increases, the wavelength λ _x increases and the wavelength λ _y decreases.

In contrast, the concave slowness (FIG. 9B) means that the absolute value of θ increases while the absolute value of both s _x and Re {s _y } increases. Thus, for concave slowness, the wavelengths λ _x and λ _y decrease with increasing θ. For the transducers shown above, the transducers in particular on piezoelectric substrates with a concave slowness are of interest.

The above waveguide is performed only in a predetermined angle range. That is, it is performed only in the region of the slowness curve R _MB between the straight lines s _x = s _{0, min} and s _x = s _{0, max} . For convex slowness, s _{0, min} = S _{0, AU} and s _{0, max} = S _{0, AB} . For concave slowness, s _{0, min} = S _{0, AB} and s _{0, max} = S _{0, AU} .

From the vertices S _{0, MB} , S _{0, AU of} the slowness curve R _MB or R _AU corresponding to the angle θ = 0, the phase velocity V _MB of the longitudinal wave in the excitation region or the external region V _MB = 1 / S _{0, MB} and V _AU = 1 / S _{0, AU} can be obtained. Since convex slowness satisfies S _{0, AU} <S _{0, MB} (see FIG. 9A), the velocity in the waveguide characterized by the convex slowness is greater in the outer region than in the excitation region. . That is, v _AU > v _MB . Therefore, this transducer functions as a waveguide when the velocity in the outer region is higher than the acoustic track in the case of convex slowness.

Similarly, it can be derived from the relationship S _{0, AU} > S _{0, MB} obtained in the concave slowness that in this case the waveguide is formed when the velocity in the outer region is lower than the acoustic track.

で得られ、ここでΔｖ_ＲＢは、励起領域と縁部領域との間の速度差、すなわちΔｖ_ＲＢ＝ｖ_ＭＢ−ｖ_ＲＢである。ｖ_ＲＢは、縁部領域における速度である。ここで仮定したのは、励起領域においてｋ_ｙ＝０が成り立つことである。条件｜ｋ_ｙ，ＡＵ｜／ｋ_ｙ，ＲＢ≧１が満たされる場合、外部領域に進入するエネルギーが殊に少なくなる。これは上記の放物線近似において条件｜Δｖ_ＡＵ／Δｖ_ＲＢ｜≧１と同義である。ｖ_ＡＵは外部領域における速度であり、またΔｖ_ＡＵ＝ｖ_ＭＢ−ｖ_ＡＵである。 The following holds advantageously for the piezoelectric substrate on which the transducer is arranged. The wave number of the surface wave is approximately a relational expression (2πf / v _MB ) ² = k _x ² + k _y ² (1 + γ) in a direction slightly deviating from the vertical propagation direction X (for example, ± 10 ° at the maximum). ) (Parabolic approximation), where f is the operating frequency of the transducer, v _MB is the velocity of the acoustic wave in the longitudinal direction X in the excitation region, and γ is the anisotropy parameter. The slowness is concave with respect to γ <−1 and convex with respect to γ> −1. Within this approximate frame, the advantageous width W of the edge region is

Where Δv _RB is the velocity difference between the excitation region and the edge region, ie Δv _RB = v _MB −v _RB . v _RB is the velocity in the edge region. The assumption here is that k _y = 0 holds in the excitation region. When the condition | ky _{, AU} | / ky _{, RB} ≧ 1, the energy entering the external region is particularly reduced. This is synonymous with the condition | Δv _AU / Δv _RB | ≧ 1 in the above parabolic approximation. v _AU is the velocity in the outer region and Δv _AU = v _MB −v _AU .

  The excitation region in FIG. 1 is configured in the same manner as in the case of the recursive filter. However, it is also possible to configure the excitation region of the transducer at least partially as follows in the longitudinal direction. That is, as is known per se known finger transducer with interdigital fingers arranged in a periodic grid (see FIGS. 2A, 2B), or per se known split finger It is also possible to configure like a converter.

  In the modified embodiment shown in FIG. 2A, the lateral edge region RB2 is between the end of the first electrode finger E11 and the second bus E2, and the lateral edge region RB1 is the second electrode. It is formed between the end of finger E22 and first bus bar E1. Due to the long grating period and the low metallization ratio (relative to the excitation region MB), a higher speed is obtained in the edge regions RB1, RB2 than in the excitation region MB.

  In the modified embodiment shown in FIG. 2A, the lattice arrangement in the edge regions RB1, RB2 is determined by the finger arrangement, that is, the lattice arrangement in the excitation region MB. In FIG. 2B, the converter is divided in the lateral direction into partial tracks having a grid arrangement independent of each other. The excitation region MB is a central partial track, and the edge regions RB1 and RB2 are outer partial tracks of the acoustic track AS. Here, the edge regions RB1 and RB2 can be regarded as regions where the buses E1 or E2 are perforated.

  In FIG. 2B, the edge regions RB1 and RB2 are implemented as a lattice arrangement having a longer period than the excitation region MB and a smaller metallization ratio than this region. The advantage of this embodiment with independent partial tracks is that the speed in the edge region can be adjusted as desired.

  Actually, since the absolute width of the edge region cannot be selected to be arbitrarily small, it is impossible to obtain a completely rectangular lateral basic mode by inserting the edge region. Thus, in another variant embodiment of the transducer described above, the adaptation of the transducer lateral excitation profile and the lateral fundamental mode by fine tuning is performed, for example, by dividing the excitation region into a plurality of partial tracks. Such adaptation by fine adjustment is possible only in a very narrow frequency range because the shape of the fundamental mode depends on the frequency.

  FIG. 3 shows a development of a transducer in which the excitation area MB of the acoustic track AS is divided into four partial tracks TB1, TB2, TB3 and TB4 in the lateral direction. These partial tracks are electrically connected in series.

3 and 4, respectively, a part of the acoustic track AS is shown schematically on the lower side, and the corresponding excitation profile Ψ _{y of the} excitation region and the shape Φ _y of the transverse fundamental mode are shown schematically on the upper side.

All the partial tracks of the excitation region thus divided are configured so that the electrode finger structure in the vertical direction X, that is, the width, the order of connection, and the interval between two consecutive fingers are the same. The widths of these partial tracks are here preferably selected separately in the transverse direction Y. The partial track marked with the number i has a width b _i . For example, in the modified embodiment shown in FIG. 3, the center partial tracks TB2 and TB3 are narrower in the lateral direction than the portions TB1 and TB4 on the edge region side.

が成り立つ。 The voltage difference between the two electrodes of this transducer is U. The excitation intensity of the electrode finger pair in one partial track is proportional to the voltage difference U _i between the electrode fingers. U _i is inversely proportional to the capacity of the partial track, and this capacity itself is directly proportional to the width b _{i of the} partial track. here

Holds.

  Therefore, the excitation intensity in the partial track i can be adjusted or weighted as desired by changing its width. When the partial tracks are connected in series, the impedance of the acoustic track AS where the excitation region is divided is correspondingly larger than the impedance of the acoustic track where the excitation region is not divided.

  In order to maintain the impedance of an acoustic track divided into partial tracks, it is possible to connect some of the partial tracks in series with each other and to connect this series circuit to another partial track or tracks. Is connected in parallel. For this, see for example the embodiment shown in FIG.

  The excitation area MB is divided into a central partial track MT and two edge partial tracks RT1, RT2. The edge partial tracks RT1 and RT2 are connected in series to each other, and a series circuit including the partial tracks RT1 and RT2 is connected in parallel to the central partial track MT. The width of the central partial track MT is significantly larger than the width of the respective edge partial tracks RT1, RT2, and is preferably at least 5 times larger. The impedance of the acoustic track AS is substantially determined by the impedance of the partial track MT configured to be relatively wide. By dividing the voltage U applied between the edge part tracks RT1 and RT2 connected in series, compared to the central part track MT to which the voltage U is applied, in each edge part track RT1 or RT2. The excitation intensity can be reduced.

  FIG. 5 schematically shows another variant embodiment of the converter described above. The figure shows a part of the transducer (bottom), the corresponding transverse fundamental mode, and the square of the transverse wavenumber (upper) depending on the lateral coordinate.

In this modified embodiment, another acoustic track AS ′ is provided, and this other acoustic track AS ′ is divided into an excitation region MB ′ and edge regions RB1 ′ and RB2 ′ in the same manner as the acoustic track AS. And substantially the same as the acoustic track AS. In this embodiment, the acoustic tracks AS and AS ′ are electrically connected in series with each other, and these acoustic tracks are arranged parallel to each other in the lateral direction. An intermediate region ZB is arranged between the acoustic track AS and another acoustic track AS ′. Edge region of from acoustic track AS AS 'RB1, RB2 and RB1', width RB2', the absolute value of _{k y} in the intermediate region ZB is significantly than the edge area RB1, RB2 and the external area AU1, AU2 (For example, by at least one order). Order to relatively quickly attenuate the fundamental mode in the intermediate region ZB, k _y in the intermediate region is advantageously pure imaginary. For this purpose, for example, the same means as in the above external area can be taken. That is, in addition to the height of the metallization, it is possible to use a material that does not increase in thickness or decrease in rigidity compared to the excitation region.

  It is possible to connect acoustic tracks arranged in parallel to each other in parallel. When two or more acoustic tracks are arranged in parallel, it is possible to combine the series connection and the parallel connection of the tracks.

In each further acoustic track of the transducer configured in a multitrack form, edge regions with (k _y ) ² > 0 are provided, in which no sound waves are excited, but corresponding excitations Waves excited in the region can propagate in the vertical direction. Between the two acoustic tracks, an intermediate layer of one having imaginary k _y is provided. In these intermediate regions, excitation of sound waves is not performed. Each intermediate region is advantageously configured as an overall metal stripe with a greater layer thickness compared to the excitation region and / or using a material with a greater thickness or less rigidity than the excitation region. Composed. The electrode fingers in the excitation region can be arranged periodically or form a unidirectional radiation cell.

  The shape of the transverse fundamental mode in which the deviation is substantially constant in the region corresponding to the excitation region and the deviation disappears in the intermediate region can be adjusted by appropriately selecting the absolute width of the edge region. Here, the width of the edge region measured by the wavelength is always 1/8 wavelength to 1/4 wavelength. In this way, the shape of the transverse fundamental mode is adapted to the excitation profile of the multitrack arrangement.

7 and 8 show that high-order transverse waveguide modes are suppressed in a transducer configured on a substrate having a concave slowness such as 42 ° rotated YXLiTaO ₃ . These higher-order wave modes are responsible for unwanted sub-maximum in the resonator admittance or filter function, and their phase coefficients depend on the lateral coordinates, and the upper curves 11, 12, 13 in FIG. In addition, their relative intensities are shown schematically on the lower side of FIG.

  A transverse mode of order 1 is a transverse fundamental mode, which is sinusoidal in an acoustic track that is conventionally constructed (with an excitation region but no edge region). This mode is indicated by curve 11 in FIG. The relative intensity of the first transverse mode is about 84%. Furthermore, in the acoustic track thus configured, another transverse wave mode having an odd order is excited. Standing sound waves corresponding to the second transverse wave mode (curve 12) are not excited in the waveguide due to symmetry conditions.

  The relative intensity of the third transverse wave mode (second harmonic of the fundamental mode, see curve 13 in FIG. 7) is now about 9%, and the fifth wave mode not shown in FIG. The relative intensity of is about 3%.

  Therefore, input coupling of electric signals to the third and fifth transverse modes is performed. This is because the transverse excitation profile of the sound wave is rectangular while the shape of the transverse mode is sinusoidal. These modes result in undesirable resonances above the pass region of the filter, and these resonances degrade the filter quality (especially the insertion loss in the pass region).

  When the excitation profile and the shape of the transverse fundamental mode are adjusted to each other, higher order transverse wave modes are not excited.

  The phase coefficients of transverse waveguide modes that can be excited or propagated in an acoustic track constructed according to FIG. 1 are shown above FIG. 8, and the relative intensities of these modes are shown below FIG. ing. The phase coefficients of the first, second and third transverse modes correspond to the curves 11 ', 12' and 13 '. The relative intensity of the higher order transverse modes is extremely small compared to the intensity of the transverse fundamental mode.

  Curves 14 and 14 'in FIGS. 7 and 8 represent the velocity profile of the waveguide corresponding to each acoustic track, where the velocity is the wave propagation velocity in the longitudinal direction. FIG. 8 shows that the wave propagation velocity in the edge region of the acoustic track is greater than in other regions of the waveguide.

  The above converter can basically be used for all SAW elements known per se, for example double mode SAW filters, ordinary finger converters, recursive filters and the number of components shown in the drawing. Or, it is not limited to a predetermined frequency region.

FIG. 4 shows the transducer, the change in wave number in the transverse direction, the corresponding fundamental mode shape and the transverse velocity profile. It is a figure which shows the converter with which the edge part area | region is formed between the edge part of an electrode finger, and the bus line which opposes this. FIG. 5 shows a transducer in which the edge region is configured as a perforated busbar region. FIG. 5 shows another transducer (lower) in which the excitation region is divided into partial tracks connected in series with each other, the corresponding lateral excitation profile, and the shape of the transverse fundamental mode (upper). FIG. 4 shows another transducer (bottom) in which the excitation region is divided into partial tracks connected in series and parallel to each other, the corresponding lateral excitation profile, and the shape of the transverse fundamental mode (top). It is a figure which shows another converter (lower) to which the some acoustic track is connected in order, the corresponding horizontal direction fundamental mode, and the change (upper) of the wave number in a horizontal direction. FIG. 4 shows a lateral profile of metallization height in a transducer with a thicker outer region compared to the excitation region. FIG. 4 shows a lateral profile of metallization height in a transducer with an outer region thickened by an additional material layer. FIG. 5 is a diagram illustrating the deviation of a transverse wave mode that can propagate in an acoustic track when the transverse excitation profile is not adapted and the excitation intensity corresponding to that mode. FIG. 6 shows the deviations of the transverse wave modes that can be propagated in the acoustic track when the transverse excitation profile is adapted to the fundamental modes and the excitation intensity corresponding to these modes. It is a figure which shows the slowness curve in the waveguide body which has convex slowness. It is a figure which shows the slowness curve in the waveguide body which has concave slowness.

Explanation of symbols

  AS acoustic track, AS 'separate acoustic track, MB acoustic track excitation area, MB' separate acoustic track excitation area, RB1, RB2 acoustic track edge area, RB1 ', RB2' separate acoustic track edge Region, E1, E2 first and second busbars, Y transverse direction, X longitudinal direction, AZ1 excitation type cell, RZ1 to RZ3 reflection type cell, AU1, AU2 external region of waveguide, TB1 to TB2 partial track, MT center Partial track, RT1 edge partial track, ZB middle region, 11 phase coefficient of lateral fundamental mode depending on lateral coordinate (if not adapted lateral excitation profile), 12 (adapted lateral excitation profile) The phase coefficient of the first harmonic of the transverse fundamental mode (if not) The phase factor of the second harmonic of the transverse fundamental mode (when the field is not adapted), 11 ′ the phase factor of the transverse fundamental mode depending on the transverse coordinates (when the transverse excitation profile is adapted), 12 'phase factor of the first harmonic of the transverse fundamental mode (when the transverse excitation profile is adapted), 13' second harmonic of the transverse fundamental mode (when the transverse excitation profile is adapted) Phase velocity of the waveguide, 14 velocity profile of the waveguide in which the fundamental mode is not adapted to the excitation profile, 14 ′ velocity profile of the waveguide in which the fundamental mode is adapted to the excitation profile

Claims (23)

In transducers operating on surface acoustic waves,
The converter
-It has an acoustic track (AS) through which the sound waves characterized by the transverse fundamental mode can propagate;
The acoustic track (AS) is divided in the lateral direction (Y) into one excitation region (MB) and two edge regions (RB1, RB2);
-Said acoustic track (AS) is arranged laterally between two external areas (AU1, AU2), said external area being adjacent to said acoustic track (AS);
-The longitudinal phase velocity v _RB of the acoustic wave in each edge region (RB1, RB2), which constitutes the edge region (RB1, RB2), is the longitudinal phase velocity of the wave in the excitation region (MB) v _Be larger than _MB ,
- constitute the external area (AU1, AU2), as longitudinal phase velocity _{v AU} of a sound wave in the external area (AU1, AU2) is smaller than _{v MB,}
-Constituting the edge region (RB1, RB2) and the outer region (AU1, AU2),
(K _y ) ² > 0 in each edge region (RB1, RB2) and (k _y ) ² <0 in each outer region (AU1, AU2)
- where _{k y} is the wave number of the transverse fundamental mode in each region (MB, RB1, RB2, AU1 , AU2),
The width of said edge region (RB1, RB2) is adjusted with reference to the width of the excitation region (MB) so that _ky is substantially constant in said excitation region (MB); The absolute value is at least 10 times smaller than the edge region (RB1, RB2) and the outer region (AU1, AU2),
A transducer that operates on surface acoustic waves. K _y ≈0 holds in the excitation region (MB).
The converter according to claim 1. The transducer is arranged on a piezoelectric substrate;
The piezoelectric substrate is selected, and (2πf / v _MB ) ² = k _x ² + k _y ² (within an angular range centering on the propagation direction (X) with respect to the velocity v _MB of the surface wave in the excitation region. 1 + γ)
So that
Where f is the operating frequency of the converter, k _x is the wave number in the longitudinal direction, γ is the anisotropy parameter of the substrate, and γ <−1 holds
The width W of the edge regions (RB1, RB2) is substantially λ _y / 8 ≦ W ≦ λ _y / 4,
-Where λ _y is the wavelength of the wave propagating laterally, where

Holds,
The converter according to claim 1 or 2. The external region (AU1, AU2) is configured so that the phase velocity of the sound wave is smaller by at least 3% than the excitation region (MB) in the external region.
4. A converter as claimed in any one of claims 1 to 3. The lateral width of each of the edge regions (RB1, RB2) is between π / (4ky _{, RB} ) and π / (2ky _{, RB} ),
Where ky _{, RB} is the fundamental mode wavenumber in each edge region,
5. The converter according to any one of claims 1 to 4. Lateral width of the outer region (AU1, AU2) of said is at least lambda _x,
Where λ _x is the wave number in the main propagation direction X in the excitation region (MB).
6. The converter according to any one of claims 1 to 5. -Having a first electrode finger connected to the first busbar and a second electrode finger connected to the second busbar;
The first and second electrode fingers are engaged with each other;
The first outer region (AU1) includes at least a portion of the first busbar;
The second external region (AU1) includes at least a portion of a second busbar;
7. The converter according to any one of claims 1 to 6. Each of the external regions (AU1, AU2) is configured as a metal stripe that extends in the vertical direction,
The height of the metal stripe is larger than the thickness of the electrode finger in the excitation region (MB).
The converter according to claim 7. The outer regions (AU1, AU2) each have at least two partial layers of different materials;
9. A converter as claimed in any one of the preceding claims. At least one of the partial layers arranged in the outer region (AU1, AU2) is thicker and / or less rigid than the electrode finger material in the excitation region (MB),
10. The converter according to any one of claims 1 to 9. The first edge region (RB1) extends between the end of the first electrode finger and the second busbar;
The second edge region (RB2) extends between the end of the second electrode finger and the first busbar;
The converter according to claim 7. The first edge region (RB1) is configured as a perforated region of the first busbar;
The second edge region (RB2) is configured as a perforated region of the second busbar;
The converter according to claim 7. The excitation region (MB) is divided into a plurality of partial tracks (TB1, TB2, TB3, TB4) in the lateral direction (Y),
The partial tracks correspond to partial converters connected in series and / or in parallel with each other,
The converter according to any one of claims 1 to 12. The partial tracks have the same electrode finger structure in the propagation direction (X);
The width of the partial track was selected so that the transverse profile Ψ _y of the excitation intensity in the excitation region (MB) matched the shape Φ _y of the transverse fundamental mode,
The converter according to claim 13. Whereas the excitation intensity transverse profile Ψ _y is adapted to the transverse fundamental mode shape Φ _y ,

Holds,
15. A converter according to claim 13 or 14. The partial track has a central partial track (MT) and two edge partial tracks (RT1, RT2);
The edge part tracks (RT1, RT2) are connected in series to form a series circuit;
The series circuit is connected in parallel to the central partial track (MT);
The width of the central partial track (MT) is at least 5 times larger than the width of each edge partial track (RT1, RT2);
The converter according to any one of claims 13 to 15. The ratio of the metallization plane in each of the edge regions (RB1, RB2) is smaller than the excitation region (MB).
The converter according to any one of claims 1 to 16. The proportion of the metallization plane in each of the outer regions (AU1, AU2) is larger than that of the excitation region (MB).
The converter according to any one of claims 1 to 17. The edge regions (RB1, RB2) have a substantially periodic metal stripe arrangement,
The period of the metal stripe is larger than the period of the electrode fingers in the excitation region (MB).
The converter according to any one of claims 7 to 18. The said excitation region (MB) is divided into cells that radiate or reflect in the longitudinal direction, unidirectionally,
A plurality of electrode fingers arranged side by side form cells that emit sound waves or cells that have a reflective action in an advantageous direction in the excitation region (MB);
The converter according to any one of claims 1 to 19. In addition to the first acoustic track (AS), at least one further acoustic track (AS ′) is provided, which further comprises an excitation region (MB ′) and an edge region (RB1) ', RB2') and is configured substantially the same as the first acoustic track (AS),
-Said acoustic tracks (AS, AS ') are arranged parallel to each other;
-An intermediate zone (ZB) is located between the two acoustic tracks;
- transverse wave number _{k y} in the 'excitation region (MB, MB) of the different acoustic track (AS, AS)' is at least 10 times smaller than the intermediate region (ZB),
21. The converter according to any one of claims 1 to 20. The excitation region, each external region (AU1, AU2), and each edge region (RB1, RB2) are configured,
(V _MB −v _AD ) / (v _RB −v _MB )> 1
So that
The converter according to any one of claims 1 to 21. In the filter,
23. A filter comprising at least one converter according to any one of claims 1 to 22.

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